Sediment driven convection below river plumes: implications to deposition patterns in lakes and estuaries

The major pathway of sediments to lakes or the coastal ocean is from sediment laden rivers. In this talk I will describe how the process of convective sedimentation could greatly increase vertically settling rates, and how this process determines that length and time-scales over which deposition can occur. Using laboratory experiments and theory we show how the speed of the plume increases with the net density difference between the layers, while in contrast, the time-scale of the settling-driven convection is related inversely to the density anomaly due to the sediment concentration difference. These competing effects result in the length-scale of propagation that increases the ratio of density differences due to salt and sediment. When this ratio is close to one, settling-driven convection is vigorous and the propagation length-scale is very small. Beneath the surface plume, the descending sediment laden fluid can also form an interflow or underflow, which is able to propagate greater distances than the surface plume. The implication of these results for sediment-laden river plumes in lakes and the coastal ocean are discussed with examples from the Squamish River in Canada and the large meromictic Lake Kivu in Rwanda Africa.